A circuit of this type can be used for example in power supply units and permits the production of stabilized output voltages, which are galvanically decoupled through the use of a push-pull current transformer with respect to the input direct voltage. It is now possible to operate such a circuit so that the feed of the electric output produced by the longitudinal regulating member occurs in the push-pull current transformer according to the principal of a current source feed. This means that at the feed point of the push-pull current transformer a capacitive energy storage does not occur so that the longitudinal regulating member does not have the characteristics of a voltage source with respect to the push-pull current transformer but instead has the characteristic of a current source. Through this principal it is possible to prevent the switch elements which exist in the push-pull current transformer from being destroyed by uncontrolled peak currents during short circuits, saturation of the push-pull transformer or overlapping operation. However, in spite of this, a certain danger of destroying the switch elements is still existent in the push-pull current transformer when, due to the current-sourcelike feed at the push-pull current transformer, inductive cut-off voltages of uncertain value, caused by the leakage inductance of a transformer which is provided in the current transformer, are created which are so high that they cause a destruction of the switch elements, for example of the transistor switches.
It is now possible to avoid also these disadvantages by connecting in parallel with the push-pull current transformer a RC-member in association with a diode in order to limit possibly occurring excessive voltages to nondamaging values. A current which is produced through such voltages is thereby bled away through the RC-member. However, it has been noted that in the ohmic resistance of the RC-member a very high output is consumed, which occurs when particularly very large leakage inductance exists in the push-pull transformer. In particular, the leakage inductance of the push-pull transformer determines this loss output very substantially because the time, during which a voltage superelevation can occur at the push-pull transformer, is determined by this leakage inductance and this time in turn determines the center current flow in the ohmic resistance of the RC-member. If such a circuit is, for example, used in a cycled current source in which exists an unstabilized direct voltage of 200 volts, a frequency of 30 kHz., a switch current of 4 amps and a leakage inductance of the push-pull transformer of 40 Micro-H., then it is already possible at a still permitted excessive voltage of 10% of the mentioned direct voltage for the loss output in the ohmic resistance of the RC-member to reach a value of more than 200 watts.
The purpose of the invention is to provide an improved solution for the protection of the push-pull current transformer with respect to uncontrolled excessive voltages through which the loss output is substantially reduced on switch elements which are to be additionally inserted.
A circuit of the abovementioned type is constructed to solve this purpose inventively so that the push-pull current transformer is bridged, if necessary, together with an inductor by a series connection of an impedance coil and a capacitor and that voltages which are symmetrically derived from its push-pull transformer are conducted each through a diode to the capacitor.
Through the invention, it is possible to dimension the voltages which are derived symmetrically from the push-pull transformer so that upon occurrence of an excessive voltage at the push-pull transformer, the respective diode receives a change of its switching condition which causes a current to flow to a connecting point between the capacitor and the impedance coil. However, through this in turn a voltage limitation is caused because through the respectively conducting diode there exists at the points of the push-pull transformer, at which the auxiliary voltages are derived symmetrically, practically the voltage which exists also at the connecting point between the capacitor and the impedance coil. Since these two elements are connected in parallel with the push-pull current transformer, in a very simple manner this connecting point can conduct the voltage of the feed point of the push-pull current transformer, so that a superelevated voltage can practically no longer occur at the push-pull transformer. Furthermore with this is associated the advantage that the pulsating currents which flow through the two diodes are smoothened by the capacitor and, due to the connection to the feed point, are returned to same so that a power loss practically no longer occurs.
A further development of the invention is characterized by the push-pull transformer being provided with an additional push-pull winding preferably in a fixed coupling. The ends of the winding are connected through the diodes to the connecting point of the capacitor and the impedance coil which is connected to the inductor. In this embodiment, the two partial windings of the additional push-pull winding can be dimensioned in such a manner that the voltage occurring on them corresponds exactly with the center voltage, which occurs at the feed point of the push-pull current transformer. If the impedance coil is connected directly to the inductor which is connected in front of the push-pull current transformer, then this center feed voltage controls, through the impedance coil, also the capacitor so that in the case of voltage superelevations through the diodes and the impedance coil, the described feeding back of the currents caused by the voltage super-elevation is possible. The fixed coupling is advantageously provided in order to cover with the additional push-pull winding as much as possible the entire leakage inductance of the push-pull transformer.
In the aforedescribed embodiment it is advantageous to connect the impedance coil directly to the inductor which connection is connected to the push-pull transformer, which results in a particularly small waviness of the current in the impedance coil. However, it is also possible to connect the impedance coil to the other end of the inductor, however, hereby the current waviness in the impedance coil is greater.
A further development of the invention is characterized by the push-pull transformer having at its primary winding, which feeds the associated switch elements, taps, which are connected through the diodes to the connecting point between the capacitor connected to the inductor and the impedance coil. In this embodiment an additional push-pull winding is not needed, but the taps of the secondary winding of the push-pull transformer are placed in such a manner that the center feed voltage occurs thereat. If the capacitor is hereby connected to the inductor, which is connected in front of the push-pull current transformer, then in the case of voltage elevations, also pulsating currents flow through the two diodes, which currents are returned to the feed point of the push-pull current transformer and are smoothened by the series connection of the capacitor with the impedance coil.